Nature vs. Nurture

I've been puzzling over this conondrum lately. Remember that old limerick:

Twixt the optimist and the pessimist
The difference is droll.
The optimist sees the doughnut
While the pessimist sees only the hole.

What makes some of us essentially happy, outgoing, content with our lot and others perpetually dissatisfied. There's a brand new baby boy next door, five days old. I haven't seen him yet but I've seen adorable pictures, and I wonder what genes, what predispositions he brings to this world. He will grow up in a loving, supportive, strong family. But what will he be?
I think of Jacob with his irrepressible happy spirit, sometimes mischievous, occasionally manipulative and pouty but generally one of the world's happy people. He is by all means a most loved child, beloved by his parents, grandparents, and extended family. Sure, without that warm environment he might be someone else, but then again his innate spirit might triumph.
I could cite examples with all of my children and grandchildren but I'd be sure to tromp on someone's toes. Still, there's Edie, who seems to have some sort of second sight or insight--I like the word fey--and as one of her uncles said when she was still an infant, "She looks at us like she has it all figured out and we don't have a clue." That's clearly something that she brought into the world with her.
As most people know, my four children are all adopted, so they clearly brought different genetic make-ups into the world. But they were raised in the same household, same environment. They are all kind, compassionate, happy people, though each different in his or her own way. What accounts for that? Surely I didn't expect them to be carbon copies of each other, and they're not.
We all know people who self-destruct. There was the suicide of a young man in our church this week, a boy who seemingly had all going for him, good family, popularity, etc. What made him so unhappy? On the other hand, we know people in dire circumstances who rise above them--homeless people who put their lives together, get an education, and become contributing citizens. Handicapped people who find some way to continue to be part of the world and to contribute to it. Culture or genes?  Look now at the people of Japan who are so valiantly working to overcome the disasters that have befallen them--sure, a lot of that is culture, but all of it?
I have no scientific evidence, no proof, but I don't believe in Rousseau's blank slate. I think each of us is born with some codes embedded in us that shape our personalities and our future. Yes, environment, family, love, all that makes a difference in shaping us. But it can't always triumph over what we bring into the world with us. The question is, can we change our inherent traits? Can we look at ourselves and say, "I'm never happy, and I don't want that. I'm going to change it."?
What about you? Is your glass half empty or half full?